1. Field of the Invention
This invention relates to a device and method for improving extrusion, particularly but not exclusively for easily oxidized metals, especially aluminum and aluminum alloys.
2. Description of the Related Art
In the process of extruding aluminum, a ram is pushed from a hydraulic cylinder against a billet of aluminum which is slidably held within a container that is heated to high temperatures.
The heated aluminum billet is, consequently, pushed through a die that is housed in a die ring and supported by a bolster, through the bolster, and through a platen to provide the desired shape for the extruded piece or pieces of aluminum.
At the high temperature of the aluminum, any contact of the surface of the aluminum with oxygen produces an aluminum oxide on the surface of the extruded piece or pieces of aluminum which degrades the quality of the finished product.
To reduce the possibility of such oxidation, an inert gas, especially nitrogen, has traditionally been pumped into the die ring and from the die ring into the cavities within the die which assist in producing the desired shape of the aluminum piece or pieces. Also, the speed at which the aluminum is pushed through this process of extrusion is maintained low in order to enable the aluminum piece or pieces to cool significantly before they exit the platen.
U.S. Pat. No. 3,808,865 provides, in lines 34 through 47 of column 2, "As has been mentioned already in connection with the known extrusion methods, the formation of an oxide layer at the downstream end of the tool, i.e., at the end of the working surface, is increased by the presence of atmospheric oxygen and by an increased deformation heat in the tool. By the total displacement of the atmospheric oxygen the formation of the oxide layer can suitably be avoided, so that with the optimum removal of heat an additional increase of extrusion speed with tolerance of an increase tool temperature becomes possible. Accordingly the invention can be advantageously embodied in that gaseous nitrogen is directed from the tool in the direction towards the downstream end of the tool."
In lines 53 through 57 of column 3 and line 65 of column 3 through line 9 of column 4, U.S. Pat. No. 3,808,865 explains that "t!he tool 16 consists, in the direction of flow of the metal which is flowing through extrusion pressure, of the die 28 which is provided with a mandrel 29 fixed to a bridge . . . The die 28 with insert 32 are set in a holder 33, which abuts through a sealing ring 34 on a pressure ring 35, which in turn is received in a pressure ring holder 36. In this case a channel shaped recess 37 is provided . . . , extending annularly in the pressure ring 35, from which passages 38 open near the downstream end of the working surface 31. In the recess 37 open two delivery pipes 39 for production of a uniform removal of heat from the tool 16, and the coolant liquid nitrogen which goes from a liquid to a gaseous state! supplied flows through the outlet opening 40 into the respective cavities 41 and 42 of the pressure ring 35 and pressure ring holder 36 . . . "
Similarly, line 67 of column 2 through line 2 of column 3 in U.S. Pat. No. 5,133,126 state that " . . . an inert gas, for example, nitrogen gas or argon gas is blown into the extrusion die in the invention because the surface of the aluminum . . . might otherwise become oxidized at high temperature . . . "
Contrary to the suggestion in U.S. Pat. No. 3,808,865, however, oxygen is not totally displaced throughout the portion of the extrusion press following the die.
Three other patents protect from oxidation only the inner surface of a tube. These are U.S. Pat. Nos. 4,316,373; 4,578,973; and 4,860,565.
From line 66 of column 3 through line 2 of column 4, U.S. Pat. No. 4,316,373 provides, "To extrude a billet 11 into a tube 17, the tip of the mandrel 16 is pressed through the billet 11, which is provided with a center bore, into the opening of the die 7 and in doing so widens the bore and centers the billet." The patent continues, "Protective gas is conveyed through the inner pipe 25 through a feed pipe 29 from the back of the mandrel holder carrier 14 to the mandrel tip 27 and flows through a bore 30 into the inside of the copper tube 17 being formed in the extrusion process."
Lines 59 through 62 of column 2 in U.S. Pat. No. 4,578,973 state, "A!n inert gas alone or a mixture of oxygen and inert gas may be supplied to the hollow portion of the tube! from the start of extrusion of the shaped material." Then, on lines 43 through 47 of column 3, the patent clarifies, "The male die 4 is centrally formed with a gas injection outlet 8. A gas channel 10 extending from a gas inlet 9 in the lower end of the die holder 6 to the outlet 8 is formed in the male die 4 and the die holder 6." U.S. Pat. No. 4,860,565 is identical to U.S. Pat. No. 4,578,973 except that it employs dry air or pure oxygen instead of an inert gas or a mixture of oxygen and inert gas.
Rather than pumping an inert gas into the die ring and from the die ring into the cavities within the die which assist in producing the desired shape of the aluminum piece or pieces, the method of U.S. Pat. No. 4,499,708 involves enclosing the entire extrusion press within a container of inert gas. Although minimizing the possibility of ignition of the reactive material with which that method deals, it would be too cumbersome for the commercial production of ordinary metals such as aluminum.
Two further patents--U.S. Pat. Nos. 5,054,303 and 5,392,628--use an opposite technique to the insertion of an inert gas. In the area around the ram, the billet, and the die, the devices of these two patents create a vacuum. Such a vacuum does not, however, extend beyond the die into the bore of the extrusion press.